fix: wrong num_entities used when mmap variable length data (#30848) (#31274)

https://github.com/milvus-io/milvus/issues/30728
pr: #30848

Signed-off-by: longjiquan <jiquan.long@zilliz.com>

internal/core/src/mmap/Column.h

@@ -326,10 +326,10 @@ class VariableColumn : public ColumnBase {
             indices_ = std::move(indices);
         }
 
+        num_rows_ = indices_.size();
+
         // for variable length memory mode only
         if (data_ == nullptr) {
-            num_rows_ = indices_.size();
-
             size_t total_size = size_;
             size_ = 0;
             Expand(total_size);